The present invention generally relates to the field of position sensing devices. In particular it relates to devices that provide an absolute measure of the rotation angle of one movable member relative to a frame of reference on a fixed member.
Many devices have been developed to measure the rotation angle of movable members. Some of these devices, such as resistive potentiometers, depend on mechanical contact between a movable part and a fixed part. Other devices use a non-contact interaction to interrogate a moving code wheel. Examples of non-contacting interactions include magnetic field sensing, capacitance sensing, and optical intensity sensing.
Code wheel devices come in two variations: incremental and absolute. Incremental devices generally have a single, binary track on the code wheel. The non-contact sensor counts the binary transitions on the track as the code wheel rotates, thereby estimating the change in angular position. Of course, if a count is missed or the power is interrupted, the estimate of angular position becomes unreliable. In an absolute device the code wheel typically has either multiple, concentric binary tracks or a single complex track in which the location around the wheel is encoded as a multi-bit word. Another alternative for encoding the absolute angle is to create a feature on the wheel whose radius is a function of angle, as disclosed in U.S. Pat. No. 5,506,579. With measurements of the radius of this feature, the angle of the wheel can be calculated. This calculation, however, may be in error because of misalignments between the wheel and the sensors.
There remains a need for methods and apparatus that provide improved measurements of the angular displacement between two members.